1. Field of the Invention
This invention is concerned with fabrication of locally distributed electrodes that are particularly useful in smart windows for controlling the reflectance and transmission of electromagnetic radiation.
2. Description of the Related Art
Smart windows are designed to reduce the amount of energy consumed for climate control of buildings and transportation vehicles by controlling the amount of solar radiation which is transmitted into such buildings and vehicles, which produces interior heating via the greenhouse effect. However, the electrochromic smart window devices which are known in the prior art have narrow dynamic ranges and involve light absorption in operation, resulting in heat being generated and transferred into the interior space by conduction, convection and infrared radiation. In addition, electrochromic devices typically utilize a relatively slow ion insertion electrochemical process that limits switching speed and cycle life. Heating of electrochromic devices by light absorption further reduces the device lifetime. Other types of smart windows, such as liquid crystal and suspended particle devices, also have limited dynamic range and typically have the added disadvantage of requiring a continuously applied voltage to maintain a given transmissive state. Consequently, an important need has developed for a durable, low-power smart window with reflectivity variable over a wide range. A smart window device based on light reflection would be much more efficient at preventing interior heating.
U.S. Pat. Nos. 5,923,456 and 5,903,382 to Tench et al. describe a reversible electrochemical mirror (REM) smart window device that provides the adjustable light reflection, wide dynamic range, long cycle life and low power requirements needed for a high efficiency smart window. In a transmissive type REM device, a mirror metal is reversibly electrodeposited (from a thin layer of liquid or gelled electrolyte) on a transparent electrode to form a full or partial mirror which provides variable reflectivity. Conversely, the mirror metal is deposited on a locally distributed counter electrode (a metallic grid on glass, for example) to reduce the reflectivity and increase the amount of light transmitted. The mirror metal is preferably silver but may be another metal, such as bismuth, copper, tin, cadmium, mercury, indium, lead, antimony, thallium, zinc, or an alloy. The transparent electrode is typically indium tin oxide (ITO) or fluorine-doped tin oxide (FTO), with a thin layer of noble metal (e.g., 15 Å platinum) that serves as a nucleation layer so that suitably smooth, mirror deposits can be obtained. The transmission of visible light, with continuous variability from around 80% to complete blocking, has been demonstrated and higher transmission for some device configurations and switching conditions should be attainable. Intermediate mirror states provide good visibility and have a pleasing bluish-gray appearance. Very little voltage is required for switching REM devices, and no voltage is needed to maintain a given switched state. As described in U.S. Pat. No. 6,301,039 to Tench, the decrease in mirror electrode sheet resistance produced by deposition of mirror metal on the mirror electrode can be used to monitor the reflectance state of the REM mirror.
Commercialization of REM smart window devices has been hindered by the expense and performance of the locally distributed counter electrode, which must present a relatively small cross-sectional area to avoid excessive light blockage that would decrease the maximum transmission of the device. The current counter electrode approach is to use a grid of a noble metal (platinum with a chromium adhesion layer, for example) which is vacuum evaporated through a photolithographic mask onto a glass substrate. The photolithographic process is inherently expensive and not readily scalable to large areas. In addition, fine grid lines (<10 μm wide) are needed so as to be invisible to the eye, but grid lines of such size are prone to damage during the photoresist liftoff process, which further increases the fabrication costs. Fine grid lines also tend to produce light interference patterns that distort images seen through the window. Furthermore, grid lines, even with mirror metal deposited on them, are relatively flat so that their actual area and cross-sectional area are nearly the same. Consequently, the current carrying capability for such grids with good light transmission is very low (approximately 5–10% of that for the mirror electrode).
An alternative approach is to use a dot matrix counter electrode, which includes microscopic islands of an noble metal distributed over a layer of a transparent metallic oxide conductor (e.g., ITO or FTO) which serves as the current collector. In this case, the mirror metal is reversibly deposited on the noble metal islands. These islands could be produced in a random pattern without photolithography by sub-monolayer vacuum evaporation or sputtering, for example. Extraneous mirror metal deposition should not occur, since the potential required for metal deposition on the bare transparent conducting metallic oxide surface is generally greater than on noble metals and on typical mirror metals. For spherical islands, the surface area is roughly four times the cross-sectional area and the current carrying capability is further increased via spherical diffusion (which is significantly faster than planar diffusion). Sufficiently small islands would not be visible to the naked eye.
In practice, however, electrodeposition of silver is initiated at defect sites on bare ITO and FTO surfaces when the applied potential is less negative than that required for deposition on the bulk materials. The defect sites (probably associated with grain boundaries) are present at relatively low density, so that silver deposits produced at moderate potentials on bare ITO and FTO are not visible to the naked eye, even after passage of an amount of charge that would yield a highly reflective mirror on the platinized surfaces. Silver electrodeposition on bare ITO and FTO is also relatively irreversible, in the sense that the deposit is not readily stripped anodically from the surface (as indicated by slow decay in the anodic stripping current) and some of the deposited silver is permanently lost (as indicated by an anodic stripping charge that is less than the metal plating charge). This indicates that the electrical/mechanical connection of silver deposits to the ITO/FTO defect sites is weak, so that some of the deposited silver flakes off. Deposition of non-adherent silver at such defect sites must be effectively suppressed to enable long-term functioning of a REM dot matrix counter electrode. The behavior of other mirror metals and other transparent metallic oxide conductors is expected to be similar.
A dot matrix electrode for which extraneous deposition on the current collector material is adequately suppressed and an economical method for making such a dot matrix electrode are needed. Such counter electrodes would be particularly useful for reducing the cost and improving the performance of REM smart window devices, but could also be used for other applications. For example, the sensitivity of electroanalyses for solution species, which is greatly enhanced by spherical diffusion at nano-scale electrodes, could be further enhanced by use of an electrode comprised of a plurality of nano-scale electrodes for which the total measurement current would be much higher.